Abstract: Water content and sulfate reducing bacteria (SRB) will accelerate the corrosion of oil tanks at bottom plate and bottom shell plate. Reason and mechanism of corrosion for internal and external surfaces of bottom plate are analyzed. Based on actual condition of the crude oil tanks in Lenghu Oilfield, effective protection measures have been taken at these locations. These measures cut down the corrosion rate of the oil tanks and thus overhaul period for the oil tanks is prolonged.